In bitstream recording one is free to subdivide the bitstream into sub-units of more regular structure. Presentation data in DVDs (digital video or versatile disc) is organised into units called Video Object Unit, denoted VOBU, e.g. in the RTRW Specification for Realtime Rewritable Video DVDs. VOBUs have a variable size (data amount measured in number of sectors), but have also a variable duration (measured in number of video fields).
For data retrieval from the disc the RTRW specification foresees a ‘VOBU map’ which is a table where for every VOBU in a recording the length in sectors and the duration in fields is entered.
EP-A-0 729 153 discloses a table that is used for trick play mode, in which table a time code is assigned to each sector on an optical disc suited for variable transfer rate.